It has been customary in the electronic article surveillance industry to apply EAS markers to articles of merchandise. Detection equipment is positioned at store exits to detect attempts to remove active markers from the store premises, and to generate an alarm in such cases. When a customer presents an article for payment at a checkout counter, a checkout clerk either removes the marker from the article, or deactivates the marker by using a deactivation device provided to deactivate the marker.
One well known type of marker (disclosed in U.S. Pat. No. 4,510,489) is known as a "magnetomechanical" marker. Magnetomechanical markers include an active element and a bias element. When the bias element is magnetized in a certain manner, the resulting bias magnetic field applied to the active element causes the active element to be mechanically resonant at a predetermined frequency upon exposure to an interrogation signal which alternates at the predetermined frequency. The detection equipment used with this type of marker generates the interrogation signal and then detects the resonance of the marker induced by the interrogation signal. According to one known technique for deactivating magnetomechanical markers, the bias element is degaussed by exposing the bias element to an alternating magnetic field that has an initial magnitude that is greater than the coercivity of the bias element, and then decays to zero. After the bias element is degaussed, the marker's resonant frequency is substantially shifted from the predetermined interrogation signal frequency, and the marker's response to the interrogation signal is at too low an amplitude for detection by the detecting apparatus.
The type of deactivation device which generates the alternating magnetic field is referred to as "active", since one or more coils are driven with an a.c. signal. The coil driving signal may have either a constant or a declining amplitude. In the former case, the marker is swept through the field to provide the requisite decaying waveform as the marker exits the field.
There have been proposed a number of coil array configurations for marker deactivation devices, including a planar array of rectangular coils (application Ser. No. 08/794,012) or "pancake" coils (application Ser. No. 08/801,489). It has also been proposed to wind the deactivation coil or coils around a magnetic core (application Ser. No. 09/016,175). These coil arrangements generate a favorable field distribution, and provide reliable deactivation of the marker even if it is presented for deactivation at some distance from the coils. However, these coil arrangements tend to be somewhat bulky and costly to produce.
It is known to provide another type of deactivator, known as "passive", and including an array of permanent magnets arranged within a housing having a very low profile. Although these so-called "deactivation pads" can fit conveniently on a check-out counter, reliable deactivation requires that the marker be brought very close to or in contact with the deactivator. This may be difficult or impossible to accomplish if the marker is incorporated in the article of merchandise or its packaging, as is done in the increasingly popular practice known as "source tagging".